1. Field of the Invention
The present invention relates to transmitting an optical polarization multilevel signal on an optical fiber.
2. Description of the Related Art
In ultra-high speed optical fiber transmission, a wavelength multiplexing transmission system is widely used in which multiple optical signals having different wavelengths are bundled for transmission in order to effectively utilize a wavelength range (or a frequency band) available for the signal transmission. In this transmission system, the transmitting side bundles and transmits multiple optical signals having different wavelengths and the receiving side receives the optical signals split into the original wavelengths, and thereby the transmission of the signals is made.
Also, to effectively utilize the frequency band, polarization multiplexing transmission is under study. The polarization multiplexing transmission system uses a multiplexing technique utilizing the difference of polarization states of light. According to the system, in the transmitting side, two pairs of optical signals modulated with an independent information signal are converted into a mutually orthogonal polarization states to be multiplexed and then transmitted to an optical fiber. The polarization state of the optical signal can be expressed as a point on the Poincare sphere surface.
By the way, it is known that the polarization state of the optical signal is subject to change during the transmission through the optical fiber. This change in the polarization state can be represented as a random conversion on the Poincare sphere surface. It is noted that the orthogonality of the polarization state is maintained after the change during the transmission. By utilizing the property, the receiving side performs a converting operation of the polarization state and a splitting operation of the polarization to split the received optical signal into two original optical signals that have been multiplexed at the transmitting side. As such, the polarization multiplexing transmission system achieves the transmission of twice as much information by using the same wavelength width as the wavelength multiplexing transmission system.
Described below is a technique for achieving the optical polarization multiplexing transmission system, that is, the optical polarization multiplexing technique at the transmitter and the optical polarization splitting technique at the receiver. First, the coherent polarization multiplexing transmission system that is one of the conventional techniques of the polarization multiplexing optical receiver will be described. P. J. Winzer, “Spectrally Efficient Long-Haul Optical Networking Using 112-Gb/s Polarization-Multiplexed 16-QAM,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 4, Feb. 15, 2010, pp. 547-556 is one of the documents disclosing the configuration of the apparatus that supports this type of transmission system.
FIG. 1, which is comprised of FIGS. 1A and 1B, illustrates an example of the conventional digital coherent polarization multiplexing transmission system. FIG. 1A represents the configuration of a polarization multiplexing optical multilevel transmitter 100 of the system and FIG. 1B represents the configuration of a polarization diversity coherent optical multilevel receiver 120 of the system.
The polarization multiplexing optical multilevel transmitter 100 splits a non-modulated laser beam output from a transmission laser source 104 into two at an optical splitter 105 and inputs them into two quadrature optical field modulators 106-1 and 106-2. The quadrature optical field modulator (or referred to as the IQ modulator) is configured with two pairs of the MZ modulators arranged in parallel on a substrate such as Lithium Niobate. In response to a fast-modulated voltage signal applied to the modulation signal input terminal of the MZ modulator, the in-phase component (the I component or the real part) and the quadrature component (the Q component or the imaginary part) of the optical field of the output light output from the output terminal can be independently modulated.
Input information signals 101-1 and 101-2 to be transmitted are encoded to multilevel signals (such as 16QAM, for example) by multilevel encoded circuits 102-1 and 102-2, respectively. The in-phase component and the quadrature component of the multilevel signal are converted into an analog electric waveform by D/A converters 103-1 to 103-4 in a fast operation and then input into the in-phase and the quadrature modulation terminals of the two quadrature optical field modulators 106-1 and 106-2.
As a result, the output lights from the respective quadrature optical field modulators 106-1 and 106-2 are turned into independent multilevel modulation lights modulated on a two-dimensional complex plane, respectively. They are converted so that the polarization states are orthogonal to each other, input to a polarization multiplexing circuit 107 as an optical modulation signal 108 of S polarization and an optical modulation signal 109 of P polarization, and output from an output optical fiber 110 as a polarization multiplexing optical multilevel signal 111.
FIG. 2A is a schematic diagram of the signal constellation according to the optical multilevel modulation and the polarization multiplexing transmission. FIG. 2A shows a sixteen-level quadrature amplitude modulation (16QAM) as an example of the signal constellation according to the optical multilevel modulation. In the 16QAM, the signal constellation is arranged in a grid-like pattern and four bits of information can be transmitted per one symbol. The example in the figure represents the values of two high-order bits (10xx, 11xx, 01xx, 00xx) on the Q axis coordinate and the values of two low-order bits (xx10, xx11, xx01, xx00) on the I axis coordinate.
Such multilevel signal can be generated by inputting the multilevel electrical signal (four-level in this example) to the input terminal of the in-phase component modulation signal and the input terminal of the quadrature component modulation signal of FIG. 1, respectively, and designating the field coordinate (i(t), q(t)) of the in-phase component and the quadrature component. At this time, the optical field of the optical modulation signal of the X polarization output from the quadrature optical field modulators 106-1 and 106-2 is expressed as (i(t)+jq(t))exp(jωt). Here, ω is an optical angular frequency of a transmission laser source 104, and j is a unit of the imaginary number. It is noted that, when a complex optical field signal is generated, the voltage signals of the real part i(t) and the imaginary part q(t) of the complex field signal may be generated using an ultra-high speed DA converter to apply them to the input terminal for the in-phase component modulation signal and the input terminal for the quadrature component modulation signal.
FIG. 2B represents the concept of the polarization multiplexing. The optical wave is one sort of the electromagnetic waves. Thus, there are two independent orthogonal polarization sates (for example, the horizontal polarization and the vertical polarization) in the optical wave depending on the vibration direction of the field with respect to its propagation direction. Therefore, two optical field components (the S polarization component and the P polarization component in the figure) can be modulated with separate information signals, multiplexed, and transmitted.
Returning to the description of FIG. 1A, the polarization multiplexing optical multilevel signal 111 output from the above-described polarization multiplexing optical multilevel transmitter 100 is transmitted through the optical fiber of, for example, a few tens to a few thousands km. In this case, the polarization multiplexing optical multilevel signal 111 is subject to transmission impairment due to chromatic dispersion and the like in the optical fiber and is received by the polarization diversity coherent optical multilevel receiver 120 of FIG. 1B. Here, the coherent reception refers to a system in which the output light of the local oscillation laser source 124 located inside the receiver is used as the reference for detection of the field component of the optical signal.
A received polarization multiplexing optical multilevel signal 121 input from an input optical fiber 122 is input to a polarization splitting optical 90-deg. hybrid circuit 125 after being appropriately amplified by an optical amplifier 123 and the like. The polarization splitting optical 90-deg. hybrid circuit 125 splits the input signal into four sets of optical signals of the X polarization components (in-phase and quadrature components) and the Y polarization components (in-phase and quadrature components) and outputs them to four balance optical receivers 126-1 to 126-4, respectively.
It is noted that the optical frequency of the local oscillation laser source 124 located within the receiver is set to substantially the same as the received polarization multiplexing optical multilevel signal 121, and its output light is connected to one input port of the polarization splitting optical 90-deg. hybrid circuit 125. The output light of the local oscillation laser source 124 is also distributed to the balance optical receivers 126-1 to 126-4 through the polarization splitting optical 90-deg. hybrid circuit 125.
In respective balance optical receivers 126-1 to 126-4, the input signal light and the local oscillation light are interfered and the lights obtained from the interference are converted into electrical signals. The electrical signals are sampled and converted into digital signals at A/D converters 127-1 to 127-4 and output to the digital signal processor.
In the digital signal processor, first, the component corresponding to the inverse function of the chromatic dispersion superimposed at the optical fiber transmission path is applied at a semi-fixed dispersion compensation circuit 128. Thereby, the waveform degradation subjected at the optical fiber transmission path is compensated. The signal in which the degradation has been compensated is provided to a polarization beam splitter 129. The polarization beam splitter 129 detects the quadrature polarization component during transmission to perform a polarization conversion, and splits and extracts the original S polarization component and P polarization component of the transmitting side. The S polarization component is output to a sampling circuit 130-1 and the P polarization component is output to a sampling circuit 130-2. In the sampling circuits 130-1 and 130-2, the data at the center time in the waveform is extracted. Next, in frequency and phase estimation circuits 131-1 and 131-2, the IF offset frequency component and the phase fluctuation component are removed. Then, in multilevel signal decision circuits 132-1 and 132-2, a decision and decoding process of the multilevel signal is performed, and output information signals 133-1 and 133-2 are obtained.
It is noted that, in general, a framer and error correction circuit is arranged in the subsequent stage of the receiver (transponder). The framer and error correction circuit analyzes the received signal to find the head of the data frame, and performs an error correction process utilizing error correction information pre-provided before the transmission, and a process of channel and monitoring information by reading out the information in the header.
Described below will be the modulation system other than the above-described polarization multiplexing transmission system in which the polarization of the optical signal is utilized. Here, the transmission system referred to as polarization multilevel modulation system in which multiple polarization states of the optical signal are utilized for information transmission will be described. The polarization multilevel modulation system is disclosed in S. Benedetto, “Multilevel polarization modulation using a specifically designed LiNbO3 device,” IEEE Photonics Technology Letters, Vol. 6, Issue 8, pp. 949-951, for example.
The principle of the polarization multilevel modulation system will be described below based on the Poincare sphere diagrams of FIGS. 3A-3D. The Poincare sphere is typically a sphere in which the radius is normalized (for example, to “1”) as illustrated in FIG. 3A and, with respect to the typical optical signal (the polarization degree is sufficiently high), the polarization state can be expressed as a point on the sphere surface. When three axes of the Poincare sphere are denoted as S1, S2, and S3 and the longitude and latitude of the signal point (the white circle) are measured as illustrated in FIG. 3A, the longitude serves as a parameter representing a manner of inclination of the polarization plane and the latitude serves as a parameter representing an ellipticity.
Each position on the Poincare sphere corresponds to the actual polarization state as seen in FIG. 3B. For example, at the intersection of the S1 axis and the sphere plane surface, the positive side represents the TE polarization (for example, the S polarization), and the negative side represents the TM polarization (for example, the P polarization). Further, on the equator (S3=0), the linear polarizations with different inclination are aligned in order. At the north pole and the south pole, the counterclockwise and the clockwise circler polarizations are arranged, and most parts on the sphere surface other than the above represent polarization states with any inclination and any ellipticity. It is noted that two points on the sphere surface located in the opposite positions interposing the center of the sphere represent the mutually orthogonal polarizations.
The multilevel polarization modulation system addressed in S. Benedetto, “Multilevel polarization modulation using a specifically designed LiNbO3 device,” IEEE Photonics Technology Letters, Vol. 6, Issue 8, pp. 949-951 is the multilevel modulation in which one polarization state of a plurality of polarization states set on the Poincare sphere is selectively transmitted. For example, FIG. 3C represents an arrangement example of the polarization states (signal points) in the case of the twelve-level polarization modulation (12PolSK). It is noted that the twelve signal points are arranged to have wide intervals between the signals and be evenly close to each other as illustrated in FIG. 3C. Each signal point on the Poincare sphere can be generated by arbitrarily modulating the amplitude and the phase of the optical field and can be used in combination of the polarization multilevel modulation and the conventional multilevel modulation.
FIGS. 4A and 4B are configuration diagrams of the conventional polarization multilevel transmission system using a digital coherent technique. FIG. 4A represents the configuration of a polarization multilevel optical transmitter 140 and FIG. 4B represents the configuration of a polarization multilevel coherent optical receiver 143.
The polarization multilevel optical transmitter 140 inputs the input information signals 101 all together into a polarization multilevel encoder (POLENC) 141 to encode them into the desired polarization state and field state. By the encoding here, generated are the optical modulation signal 108 of the S polarization component having any amplitude and phase and the optical modulation signal 109 of the P polarization component similarly having any amplitude and phase. The polarization multiplexing circuit 107 coherently multiplexes the two optical modulation signals 108 and 109 to generate any polarization multilevel and optical multilevel signal 142.
The apparatus configuration of the polarization multilevel optical transmitter 140 illustrated in FIG. 4A is substantially the same as that of the polarization multiplexing optical multilevel transmitter 100 illustrated in FIG. 1A, but different in that it is necessary to create the path lengths and/or the modulation timing in a high accuracy because of the needs of the coherent addition of the S polarized optical modulation signal 108 and the P polarized optical modulation signal 109, and therefore the configuration of the apparatus is slightly complicated.
The apparatus configuration of the polarization multilevel coherent optical receiver 143 illustrated in FIG. 4B is also similar to that of the polarization diversity coherent optical multilevel receiver 120 illustrated in FIG. 1. One of the differences is in that the polarization beam splitter 129 (FIG. 1B) is replaced with a polarization state estimation circuit 144 (FIG. 4B). The internal parts of the optical transmission apparatus and/or the optical fiber transmission path currently used do not have the mechanism for holding the primary axis of the polarization in a constant direction. Therefore, the mapping of the signal points of the received polarization multilevel signals onto the Poincare sphere exhibits a state that is subjected to the three-dimensional rotation in any direction with respect to the signal constellation at the time of transmission, as illustrated in FIG. 3D. In FIG. 3D, this state is represented as any rotation of the equator plane (the hatched part). Therefore, the polarization multilevel coherent optical receiver 143 is provided with the polarization state estimation circuit 144 as the mechanism for estimating the direction of the original polarization primary axes (S1, S2, and S3 illustrated as the dotted lines in the figure). Further, another difference is in that the multilevel signal decision circuits 132-1 and 132-2 (FIG. 1B) are replaced with a polarization multilevel decoder 145 (FIG. 4B). The polarization multilevel decoder 145 recovers the information signals all together based on the polarization state and/or the amplitude and phase of the received constellation.
Subsequently, described will be the modulation system other than the above-described polarization multiplexing transmission system in which the polarization of the optical signal is utilized. Here, the differential polarization modulation system will be described. The differential polarization modulation system is a system to transmit the information by utilizing the change in the polarization between the received symbol and the immediately preceding symbol. For example, U.S. Pat. No. 4,831,663 discloses an example of transmitting the binary information, for example.
FIGS. 5A-5C illustrate the differential polarization modulation system. In the differential polarization modulation system, as illustrated in FIG. 5A, the transmission is made so as to alternatively switch two orthogonal polarizations. FIG. 5B is a view illustrating the state transition mapped on the Poincare sphere. In this system, the digital information “1” is transmitted at the time when the polarization is switched, while the digital information “0” is transmitted at the time when the polarization is unchanged. Therefore, the receiver supporting the system demodulates the received symbol by the coherent heterodyne detection of the received differential polarization modulation light, detects the change in the phase or the amplitude by calculating the product or the difference of the received symbol and the immediately preceding symbol, and demodulates the information signal based on the detection result. The differential polarization modulation system is able to transmit the information without requiring the strict detection of the polarization state, and has advantage that it is unlikely to be subjected to the impairment even in the transmission path where the polarization state changes rapidly.
Further, J. Blaikie, etc., “Multilevel Differential Polarization Shift Keying”, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 45, NO. 1, January 1997 proposes the differential polarization multilevel system in which the number of levels is increased. In general, in the high order differential polarization modulation, there remains uncertainty in deriving, from the polarization state S(n−1) of the immediately preceding symbol only, the change of the polarization state to the polarization state S(n). This is because the rotation of the Poincare sphere in the transition from S(n−1) to S(n) is not uniquely determined. Thus, the system disclosed in J. Blaikie, etc., “Multilevel Differential Polarization Shift Keying”, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 45, NO. 1, January 1997 also utilizes the polarization state S(n−2) that is two symbols preceding in the past. In this system, a rule is defined that the same polarization or the mutually orthogonal polarizations are not transmitted in consecutive two symbols. Two decision variables d1(n) and d2(n) used in the receiver supporting the system are defined as the following equation (8) in J. Blaikie, etc., “Multilevel Differential Polarization Shift Keying”, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 45, NO. 1, January 1997.d1(n)=S(n)S(n−2)/(|S(n)∥S(n−2)|)  (Equation 1)d2(n)=S(n)S(n−1)×S(n−2)/(|S(n)∥S(n−1)×S(n−2)|)  (Equation 2)
Here, d1(n) is a normalized inner product of S(n) and S(n−2), and d2(n) is a normalized inner product of S1 and a normalized outer product vector of S(n−1) and S(n−2).
FIGS. 6A-6E illustrate the principle of the multilevel differential polarization modulation. FIG. 6A illustrates a positional relationship among three symbols S(n−2), S(n−1), and S(n) that have been received consecutively in time. The d1 provided by Equation 1 is a coordinate value of the received symbol S(n) measured along the D1 axis that passes the Origin and S(n−2) as illustrated in FIG. 6B. In the case of this example, S(n) is on the surface of d1=0. Therefore, d1=0.
On the other hand, d2 provided by Equation 2 is a normalized inner product measured along the D2 axis (the axis parallel to the outer product vector S(n−1)×S(n−2), that is, orthogonal to both S(n−1) and S(n−2)) as illustrated in FIG. 6C. In the case of this example, S(n) is just on the D2 axis and thus d2=1.
As illustrated in FIG. 6C, the D1 axis and the D2 axis both pass the Origin of the sphere and are not parallel to each other. Therefore, the position S(n) of the received symbol can be determined uniquely to the coordinates (d1, d2). As described above, the d1 axis and the d2 axis are defined from the positions of two symbols and it is therefore confirmed that (d1, d2) is a differential demodulation result with respect to the positions for past two symbols. Such multilevel differential modulation is useful, in particular, for the case where the number of the signal points is increased for the improved transmission efficiency in the polarization multilevel transmission.
Typically, in the polarization multilevel modulation system, when the number of the signal points is increased (when a large number of signal points are densely arranged on the two-dimensional Poincare sphere surface), the correct decision of the signal point will be impossible even with a slight inclination of the Poincare sphere (even with a slight rotation of the polarization primary axis). Further, in the polarization multilevel modulation system, the increased number of the signal points makes it quite difficult to detect the primary axis or track the change in the primary axis after the reception. In particular, when the degradation of the SN is larger or when a rapid fluctuation is generated in the polarization state, the error in the detection of the polarization axis or in the tracking occurs, which makes the reception impossible.
In contrast, in the case of the above-described differential multilevel polarization modulation system, the symbol decision is made based on the difference and the like in the polarization between two consecutively received symbols, so that the accurate and fast polarization tracking is not required and thus the strength against the fast polarization fluctuation is improved.